Early diagenetic alteration of sedimentary organic matter in the maruhubi mangrove stand

The contents of TC, OC, TN, CaCO3 and C/N ratios for sediments collected from Maruhubi mangrove stand were used to (i) examine the downcore diagenetic changes of OM, (ii) delineate sources of OM, (iii) assess whether there are differences in rate of OM degradation between zones by using a multi G-model of Berner (1980), and (iv) find out if there are differences in accumulation rates of OM and elemental composition among zones. The mean contents as well as accumulation rates of TC, OC, and N were highest at sites MAR 2 and MAR 3 and lowest at site MAR 1 owing probably to terrestrial inputs from land and nearby settlements, higher sedimentation rate and enhanced preservation. Moreover, these parameters exhibited a general downcore decrease with depth an indication of diagenetic alteration of OM with little influence from variability in OM input with time. Low content of CaCO3 was observed in all cores and suggests lower dominance of CaCO3 producing organisms in mangroves and/or high dilution by silisiclastic materials reworked from paleobeaches. Although the mean C/N ratio values, which ranged from 5.2 (MAR 1) to 15 (MAR 2) may suggesting preferential deposition of marine derived materials, this is unlikely as few available δ13C values suggest largely terrestrial contribution. This shows that C/N ratio is not a reliable indicator of sources of organic material in marine environment and it has to be used with caution. The degradation rates constants (k) were highest for N than C and k-values decreased seaward suggesting deposition of more refractory material at the seaward site as well as higher degradation rate of N relative to C. TOC buried in 100ha of the Maruhubi stand is 339T/yr which is equivalent to 1.59x10-7 ppmv of CO2 removed from the atmosphere.